1. Field of the Invention
The present invention relates to a power transfer apparatus for secondary or coupled driving wheels of a four-wheel drive vehicle.
2. Description of the Related Art
In conventional four-wheel drive vehicles, in turning a corner of a small turning radius at low or middle speed in a four-wheel driving mode, there is caused a difference in wheel speed between front and rear wheels which is attributed to a difference in turning radius between the front and rear wheels, resulting in the occurrence of a tight corner braking phenomenon.
Front and rear wheels driving systems disclosed in JP-B-7-61779 and JP-B-7-64219 are known as related arts for solving the problem of tight corner braking phenomenon.
In the front and rear wheels driving systems disclosed in the Japanese Examined Patent Publications, an average wheel speed of secondary driving wheels relative to an average wheel speed of primary driving wheels is adjusted by providing a transfer (a speed increasing system) between the primary and secondary driving wheels.
In this transmission, by switching on and off a direct connective clutch and a transmission clutch, there occurs a switch between a direct connecting mode in which the average wheel speed of the primary driving wheels and the average wheel speed of the secondary driving wheels are almost equalized and a speed increasing mode in which the average wheel speed of the secondary driving wheels is made larger than the average wheel speed of the primary driving wheels.
In this front and rear wheels driving system, when turning a small corner in the four-wheel driving mode, the occurrence of tight corner braking phenomenon is prevented by bringing the secondary driving wheels in the speed increasing mode by the transmission.
Incidentally, in the aforesaid transfer (the speed increasing apparatus) for four-wheel drive vehicles, at least two hydraulic or electromagnetic actuators are required as a power source for operating the direct connective clutch and the transmission clutch, and this leads to a problem that the transmission itself is made larger in size and hence heavier in weight.
Furthermore, in this transmission, since constituent components are assembled piece by piece in the assembling processes of the transmission, the number of processes is increased and hence the productivity is deteriorated. In addition, since there are many items needing adjustments such as clearance and spring load, the productivity is also deteriorated.